Electrical furnace for melting glass

ABSTRACT

An electrical furnace particularly suitable for melting thermoplastic material used in producing glass fibers includes a tank having side walls constructed of a refractory material having an electrical conductivity greater than the material being melted at furnace melting temperature. In order to minimize electrical conduction through the side walls, first and second sets of electrodes are arranged with one set centrally disposed in the tank and the other set surrounding the first. Electric power is applied between the first and second electrodes to generate a potential field with substantially continuous equipotential lines between the second electrodes so as to shield the first electrodes from the side walls. An electrical control circuit limits the current flow between any two electrodes to prevent a runaway condition which might otherwise be caused by the steep negative temperature-resistivity characteristics of the glass. The control circuit includes a circuit which minimizes the DC component from current flow between the electrodes.

United States Patent [191 Clishem et al.

[ June 18, 1974 ELECTRICAL FURNACE FOR MELTING GLASS [75] Inventors: Thomas A. Clishem, Valley Station;

Francis R. Duerr, Devondale, both of Ky.; Carl T. Snook, Utica, Ind.

[73] Assignee: Corhart Refractories Company, Louisville, Ky'

22 Filed: Apr. 30, 1973 211 Appl.No.:355,877

Primary Examiner-Roy N. Envall, Jr. Attorney, Agent, or FirmRichard N. Wardell; Norman L. Norris [57 ABSTRACT An electrical furnace particularly suitable for melting thermoplastic material used in producing glass fibers includes a tank having side walls constructed of a refractory material having an electrical conductivity greater than the material being melted at furnace melting temperature. In order to minimize electrical conduction through the side walls, first and second sets of electrodes are arranged with one set centrally disposed in the tank and the other set surrounding the first. Electric power is applied between the first and second electrodes to generate a potential field with substantially continuous equi-potential lines between the second electrodes so as to shield the first electrodes from the side walls. An electrical control circuit limits the current flow between any two electrodes to prevent a runaway condition which might otherwise be caused by the steep negative temperature-resistivity characteristics of the glass. The control circuit includes a circuit which minimizes the DC component from current flow between the electrodes.

5 Claims, 22 Drawing Figures Pmimmmww slelalllz sum 01 (1F 11 BATC H IN PATENIEmumm 3.8181112 sum 02 ur 11 PATENTEDJM 1a 1914 snm 03 0F 11 PATENTEBM 1a i914 AC INPUT VOLTAGE LOAD VOLTAGE SCR BLOCKING VOLTAGE PULSE RESISTIVIT Y PATENTEBJun 10 m4 sum 01 or 11 RESISTIVITY TE MPERATURE BOROSILICATE GLASS ODA -LlME- SILICA GLASS TEMPERATURE (c) ale-mini PATENTEDJuu 18 1914 PATENTEDJun 18 I974 saw us or 11 PAIENTEDJun 18 1914 $81 8.1 l

sum 10 or 11 PATENTEBJumwn v 3l818Ll12 sum -11nr11 1 ELECTRICAL-FURNACE FOR MELTING GLASS BACKGROUND OF THE INVENTION This invention relates to an electrical furnace and the method of operating this furnace to melt thermoplastic material suitable for production of glass fibers.

Glass produced in quantity is usually melted in a regenerative furnace fired with oil, natural gas, or other liquid or gaseous fuel. The glass is contained in a tank which is usually a rectangular bath constructed of refractory blocks. The fire is contained in a gas space above the tank.

The material fed to the tank is called batch" which is the heterogenous mixture of raw substances which when degased and fused together make the glass. The batch commonly contains a modest amount of cullet which is glass that has been melted before and is now being used over again.

Recently, electrical furnaces have been used in the production of glass. Electrical furnaces are shown in US. Pat. No. 3,524,206 Boettner et a1. and in US. Pat. No. 3,636,227 McQuaid. Electrical furnaces have advantages over fuel fired furnaces particularly with regard to lessened environmental pollution.

To our knowledge, electrical furnaces have not heretofore been successfully used in the production of glass fibers in furnaces having sidewalls with conductivity comparable to that of the thermoplastic material. The thermoplastic material which is melted as a preliminary step in the production of glass fibers is commonly referred to as E glass. A typical composition of E glass is:

SiO 54% AIQO; 14 C110 17.5 MgO 4.5 B Total IOOIWE We have identified two very significant problems in melting E glass in an electric furnace. The first is that the refractory material (eg chromic oxide) of which the furnace sidewalls are commonly constructed has an electrical conductivity which is greater than or comparable to that of the E glass at furnace melting temperature. Instead of current flowing from one electrode to another through the molten E glass to induce heating, a portion of the current flows through the higher conductivity path presented by the side walls. In addition to such decreased heating efficiency, this short circuiting effect also causes shortened life of the refractory walls.

The other problem associated with melting E glass in an electric furnace is that E glass has a very steep negative temperature resistivity characteristic. Most glasses have such a negative temperature coefficient. For example, borosilicate glass sold under the trademark Pyrex and common soda-lime-silica glass both decrease in electrical resistivity as the temperature increases. However, the decrease in resistivity with temperature is relatively small. On the'other hand, for the same change in temperature, E glass exhibits a markedly larger decreasein resistivity. Because of this, there tend to be runaway" paths between electrodes in an electrical furnace. A slight increase in current flowing directly between the electrodes increases the heating along this path. Therefore the resistance of the E glass decreases along this path resulting in an even more increased current flow along the path. The effect is cumulative producing the runaway path between the electrodes whereas of course it is desirable to distribute current flow in predetennined regions of the batch.

Various electrode arrangements and configurations have been used in prior art electrical furnaces. US. Pat. No. 2,018,886 Ferguson shows an electrical furnace with a central electrode and a second circular ring electrode which encircles it. This arrangement would operate satisfactorily if the resistance of the batch varies in a predetermined manner. However, if the resistance along one radial path was slightly low, a runaway path might develop. The electrode arrangement cannot accommodate electrical control circuitry which will insure uniform distribution of current throughout the batch.

Another electrode arrangement in a glass furnace is shown in US. Pat. No. 2,417,913 Cornelius. Cornelius shows a row of electrodes along one side wall, a row of electrodes in the middle and a row of electrodes on the opposing side wall. Three-phase power is applied between all three rows. Since a voltage is imposed between the two rows of electrodes along the sidewalls, current flow through relatively high conductivity sidewalls would be induced during melting E glass.

SUMMARY OF THE INVENTION In accordance with this invention thermoplastic material is melted in an electric furnace with an electrode arrangement which applies a potential field (i.e., electrostatic field) to the thermoplastic material which effectively shields the sidewalls from current conduction paths. More particularly, first electrodes are mounted in the center of the furnace tank. Second electrodes are mounted with predetermined spacing around the inside of the periphery of the tank. Electrical power is applied between the first and the second electrodes so that a potential field is created in the thermoplastic material with equi-potential lines extending substantially continuously between the second electrodes. This effectively shields the first electrodes from the sidewalls and minimizes current flow through the sidewalls.

In accordance with this invention the electric power is controlled by feedback circuits which minimize the possibility of a runaway path in the thermoplastic material. Such a power control circuit is provided for different sets of first and second electrodes distributed throughout the furnace. In this way a good distribution of heating current throughout the batch is obtained and the likelihood of a runaway path between any one set of electrodes is minimized.

In accordance with another aspect of this invention the feedback circuitry includes a circuit which reduces, or eliminates, the direct current component in the applied power. It has been found that this direct current component causes undesirable polarization and blister formation in the glass, excessive heating around the electrodes and accelerated deterioation of the electrodes. The control circuitry of this invention eliminates these undesirable effects.

The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended claims.

DESCRIPT ION OF THE DRAWINGS FIG. I is a view along the section 11 of FIG. 2;

FIG. 2 is a top view of the furnace;

FIG. 3 is a block diagram of the electrical control circuit for one set of electrodes;

FIG. 4 is an electrical schematic diagram of one embodiment of the control circuit;

FIG. 5 is a diagram of another embodiment of the electrical circuit;

FIGS. 6A-6E are waveforms depicting the operation of the circuit of FIG. 5;

FIG. 7 shows the resistivity-temperature characteristics of three types of glass;

FIGS. 8-l1 depict potential fields produced by various electrode arrangements in electric furnaces; and

FIG. 12 shows typical firing sequences.

DESCRIPTION OF A TYPICAL EMBODIMENT FIGS. 1 and 2 show an electrical furnace including a tank having sidewalls l0, 12, Hand 16 and a bottom 18. As is common in fumacesof this type a batch inlet and a glass outlet are'provided. First electrodes 20, 22

g and 24 are mounted in a row in the middle of the tank.

Second electrodes 26, 28, 30, 32, 34, 36, 38 and 40 are positioned in the bottom of the tank. The spacing between the second electrodes is predetermined and the second'electrodes are positioned around the inside periphery of the tank between the first electrodes and the sidewalls. A batch of thermoplastic material 42 is heated bycurrent passing between the first and second electrodes. The furnace is of the type particularly suitable for melting E glass for the manufacture of glass fibers.

FIG. 3 shows the source of electric power applied between one set of first and second electrodes to induce current flow through the glass. AC power from a source which includes the transformer 44 is applied through solid state controllers 46 which are connected in series between the electrodes 24 and 32. In this embodiment, the solid state controllers are included in a single device commonly referred to as a triac. In a preferred embodiment to be subsequently described, a pair of silicon controlled rectifiers are used to control the current. A trigger circuit 48 controls the phase angle at which each of the controllers fire during alternate half cycles of the AC power. A feedbackQcircuit 50 controls the current flow between the first and second electrodes. A shunt resistance 52 is connected in series with the electrodes. The voltage across this resistor is a measure of the current and is applied to the feedback circuit 50. The triac controllers shown in FIG. 3 were used in model studies only but this circuit illustrates the principles of operating a furnace in accordance with this invention.

FIG. 4 shows one embodiment of the trigger circuit 48 and the feedback circuit 50 for the circuit of FIG. 3. A feedback signal developed acrossresistor 52 is coupled through transformer 54 to the lamp 56. Lamp 56 produces a light intensity which is proportional to the square of the current through the feedback resistor 52. This light impinges on the photoconductive cell 58. The voltage across this photocell 58 is proportional to the mean square value of the current through the resistor 52.

The triggering circuit includes a capacitor 60 and a unijunction transistor 62. The capacitor 60 is charged by the voltage across the control device 46. When the voltage on the capacitor 60 reaches a certain level, the transistor 62 conducts. A pulse of current passes through the primary of pulse transformer 64. The secondary is coupled to the gate of the control device 46 and triggers this device.

The voltage across the control device 46 is rectified in a full wave rectifier including the diodes 66, 68, and 72. Zener diodes 74 and 76 clip peaks or excursions in the'voltage. I

The feedback circuit adds a bias voltage to the voltage applied to capacitor 60. Instead of starting atzero, the charging voltage starts at the feedback levelsupplied by the photoconductor 58.

The control action is as follows. Assume that the current through the feedback resistor is above the desired regulated value. The feedback signal applied to lamp 56 increases the-light intensity and this decreases the resistance of photocell 58. Therefore, the initial bias .voltage applied to the capacitor 60 decreases. There'- fore, it takes longer in the half cycle for thecapacitor 60 to charge to the firing voltage of the unijunction transistor 62. Therefore, the control device 46 fires later in the half cycle and the current supplied to the electrodes is decreased. 1

Typical component values for a circuit of this type can be obtained from the General Electric Transistor Manual.-

An alternate embodiment using silicon controlled rectifiers is shown in FIG. 5. Back to back SCRs 80 and 82 are connected between the AC power source and the electrodes. Two control loops are used to provide regulation. The control loop 84 regulates the rms value of load current. The control loop 86 regulates the DC component of the current between the two electrodes.

The signal from the feedback resistor which is in series with the electrodes is applied to amplifier 88. The feedback signal is summed in summing amplifier 90 with the signal representing the desired levelof AC current. This is obtained from the mean square current reference potentiometer the output of which is applied to the reference signal amplifier 91. The output of summing amplifier 90 is applied to a photomodulator 92 which may be similar to that previously described. This produces a signal proportional to rms current through the feedback resistor. A control amplifier 94 acts through gating circuits 96 and 98 to control the conduction angle of the SCRs 80 and 82 thereby regulating the AC current between the electrodes and maintaining it at a desired value set by an input potentiometer.

The other control loop 86 determines whether there is a deviation of the DC current component from the desired zero level. In order to detect such a deviation, the feedback signal from amplifier 88 is rectified in rectifier 100 and integrated in integrator 102. The same feedback signal is inverted in inverter 104, rectified in rectifier 106 and integrated in integrator 103. The difference between the outputs of integrators 102 and 103 is a measure of the DC component in the signal. In order to obtain this difference, the outputs of integrators 102 and 103 are applied to differential amplifier l 10. The resultant signal is applied to control amplifier 112. A zero adjustment of the DC component is made by the DC reference potentiometer 114. The output of control amplifier 112 acts through the summing amplifier 116 to control the firing angle of the silicon control rectifier 82. The firing angle of SCR 82 is advanced or delayed to return the DC component to zero.

The waveforms of the FIG. 5 circuit are shown in the FIGS. 6A-6E. FIG. 6A shows the AC input voltage, FIG. 6B shows the voltage across the load and FIG. 6C shows the voltage across the SCRs. The gating pulses applied to the SCRs are shown in FIGS. 6D and 6E.

The method of operating the electrical furnace to effectively melt E glass can be better understood with reference to FIGS. 7-11. FIG. 7 shows the steep resistivity-temperature characteristic of E glass as compared to a common borosilicate glass and a common soda-lime-silica glass. Because of the very sharp reduction in resistivity for a given increase in temperature, E glass is particularly susceptible to runaway current paths during electrical heating. However, by providing a plurality of current controllers each controlling the current through a given set of electrodes, the current can be well distributed in the E glass without danger of runaway.

FIG. 8 depicts the equi-potential lines of potential field formed by a voltage applied between two electrodes at the bottom and the common electrode at the top. Current flow is transverse to the equi-potential lines. Note that there is a path between the two electrodes at the bottom whereby current can flow-to the side wall and through the side wall. FIG. 9 depicts the shielding effect which is obtained when the two bottom electrodes are moved closer together. This arrangement is less susceptible to current flow through the sidewalls. FIG. 10 shows the electrostatic field formed when three common electrodes are arranged in a row in the middle and surrounded by eight electrodes to which the opposite polarity voltage is applied. The shielding effect is good.

FIG. ll depicts the effect of high conducting sidewalls on the potential field of FIG. 10. The shielding effect is still good.

It will be appreciated that the applied voltage and the number and spacing of the electrodes can be varied to provide most effective shielding for different size tanks. The equi-potential lines for any given arrangement can be plotted by using modeling techniques. Optimum operation will be achieved by producing a potential field such as that shown in FIG. 11, or a potential field with even more continuous equi-potential line extending between the outer electrodes.

The following description of one particular operating furnace is given by way of example only. The furnace side walls were constructed of refractory brick commercially available from the Corhart Refractories Company with the designation C-1 215. This refractory has a resistivity at l,500C of about 1 ohm centimeter. The resistivity of the E glass at l,500C is about 24 ohm centimeter, very much greater than that of the sidewalls. The furnace bottom is constructed of a zircon material commercially available from Corhart Refractories Company and identified as ZS-l300. It has a resistivity at l,500 of about 3,400 ohmcentimeters and does not significantly contribute to overall refractory conduction. Electrodes are Tl 185 tin oxide electrodes commercially available from Corhart Refractories Company. The controlled voltage input between each set of electrodes is 180-200 volts AC, single phase 60 Hz. The output current is regulated to plus or minus 1 percent for load currents of 20 amps to 200 amps. Silcenter electrodes and surrounding electrodes, and in some cases for sometime thereafter (e.g. until the molten glass pool reaches refining temperature approximately l,500 C. for E glass). This system can also be used in a booster type operation wherein thermal energy is supplied by both electricity and fuel.

Typical firing combinations between the electrodes are shown in FIGS. l2A-l2F. Other combinations can be used. The controllers can also be connected to control the power applied to the center electrodes instead of the outer electrodes as shown.

Other possible modifications include dividing the furnace tank into individual, controllable zones and using a different phase of a multiple phase power supply for each zone. For example, the furnace might be divided into three zones with each zone having an electrode configuration similar to that shown in FIG. 2. One phase of a three phase power supply could be applied to the center electrodes of each zone with the outer electrodes common. With this configuration zone control can be accomplished either by sequentially switching the inner electrodes or the outer electrodes. However, by switching the inner electrodes the degree of shielding remains substantially constant over the entire electrical cycle.

The foregoing and other modifications are within the scope of this invention and the appended claims are intended to cover such modifications.

What is claimed is:

1. An electrical furnace for melting thermoplastic material comprising:

a tank having side walls,

first vertical electrodes disposed between said side walls in said tank,

second vertical electrodes disposed about said first electrodes,

a source of substantially AC electric p wer applied only between sets of said first and second electrodes to induce current flow through said thermoplastic material between said first and second electrodes while minimizing current flow through said side walls, and H I a plurality of current controllers, one for each set of electrodes, each current controller controlling the current through a given set of electrodes.

2. An electrical furnace for melting thermoplastic material comprising:

a tank having side walls,

at least one first vertical electrode disposed between said side walls in said tank,

second vertical electrode means disposed about said w slsstrqsisaan V.

a source of substantially AC electric power applied only between said first and second electrodes to induce current flow through said thermoplastic material between said first and second electrodes While minimizing current flow through said side walls, and v V a regulated power supply having a feedback circuit for controlling the current flow between said first and second electrodes.

3. The furnace recited in claim 2 wherein said power supply includes solid state power controllers connected in series between an AC power supply and one of said electrodes, and i a triggering circuit for controlling the phase angle at which said solid state controllers fire during each half cycle of said AC source, said feedback circuit being connected to said triggering circuit to control said phase angle.

4.The furnace recited in claim 3 wherein said power supply further comprises:

a current feedback resistor connected in series between said solid state controllers and one of said electrodes,

a circuit producing an output indicating the direct current flow through said resistor, and

a circuit responsive to said output for changing the firing angle of one of said solid state controllers to minimize said direct current component.

5. The method of melting glass in a furnace having side walls constructed of refractory material having an electrical conductivity comparable to or greater than temperature resistivity characteristics of said glass. 

1. An electrical furnace for melting thermoplastic material comprising: a tank having side walls, first vertical electrodes disposed between said side walls in said tank, second vertical electrodes disposed about said first electrode, a source of electric power applied only between sets of said first and second electrodes to induce current flow through said thermoplastic material between said first and second electrodes while minimizing current flow through said side walls, and a plurality of current controllers, one for each set of electrodes, each current controller controlling the current through a given set of electrodes.
 2. An electrical furnace for melting thermoplastic material comprising: a tank having side walls, at least one first vertical electrode disposed between said side walls in said tank, second vertical electrode means disposed about said first electrode, and a source of electric power applied only between said first and second electrodes to induce current flow through said thermoplastic material between said first and second electrodes while minimizing current flow through said side walls, and a regulated power supply having a feedback circuit for controlling the current flow between said first and second electrodes.
 3. The furnace recited in claim 2 wherein said power supply includes solid state power controllers connected in series between an AC power supply and one of said electrodes, and a triggering circuit for controlling the phase angle at which said solid state controllers fire during each half cycle of said AC source, said feedback circuit being connected to said triggering circuit to control said phase angle.
 4. The furnace recited in claim 3 wherein said power supply further comprises: a current feedback resistor connected in series between said solid state controllers and one of said electrodes, a circuit producing an output indicating the direct current flow through said resistor, and a circuit responsive to said output for changing the firing angle of one of said solid state controllers to minimize said direct current component.
 5. The method of melting glass in a furnace having side walls constructed of refractory material having an electrical conductivity comparable to or greater than said glass comprising: introducing a batch of said glass into said furnace, applying an electrical field to said batch by applying electrical power between sets of first electrodes disposed between said walls in said tank and second electrodes disposed about said first electrodes, and regulating the current flowing between said first and second electrodes in each set of electrodes in said batch to prevent large current flow in one path caused by cumulative reduction in the resistance of said glass along said path as current cumulatively increases along said path due to the steep negative temperature resistivity characteristics of said glass. 